1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method using an electrophotographic technique and, more particularly, to a mechanism for coping with a defect occurring in a fixing step of fixing a toner image in an image forming process.
2. Description of the Related Art
A laser beam printer (hereinbelow, also referred to as an LBP) as an image forming apparatus using an electrophotographic technique has been used hitherto.
In the LBP, a photosensitive drum is uniformly charged by a charging roller. After that, a scanner modulates an intensity of a laser beam based on an image signal included in data of a print job sent from a host computer, thereby forming an electrostatic latent image onto the photosensitive drum. Toner is deposited onto the electrostatic latent image on the photosensitive drum by a developing roller, thereby forming a toner image. The toner image on the photosensitive drum is transferred onto a transfer material by a transfer roller. After that, the toner image on the transfer material passes through a conveying belt, a fixing roller, and a pressing roller and becomes a permanent fixed image. The transfer material is stacked onto a tray by a discharge roller.
In the ordinary printer, as illustrated in FIGS. 8 and 9, when a straight line image 1302 in the main scanning direction is output, the following problem would be occur. That is, in a fixing step, there is such a problem that in a line image visualized as a toner image on a transfer material 1301, the toner is scattered at a trailing edge in the sub-scanning direction (also referred to as a downstream in the sub-scanning direction), so that the image is disturbed (such a phenomenon is referred to as a “tail” illustrated at 1303 in the diagrams).
Such a phenomenon occurs because moisture in the transfer material is explosively evaporated due to a sudden temperature increase in the fixing step and a pressure by a fixing apparatus (a fixing roller 1209a and a pressing roller 1209b), the steam comes out of the trailing edge of a weak force and, at the same time, the toner is also scattered (FIG. 9). Generally, there is a correlation between the tail and a toner amount. The larger the toner amount is, the more the tail is liable to occur.
There is an offset phenomenon as another image defect which occurs in such a printer. The offset phenomenon is that when the non-fixed toner image is fixed by the fixing apparatus, the toner image is electrostatically transferred to the fixing roller 1209a and the toner image is fixed to another portion of the transfer material 1301 and becomes an offset image. Also with respect to the offset phenomenon (also simply referred to as an offset hereinbelow), in a manner similar to the tail, the larger the toner amount is, the more the offset is liable to occur.
As a method of solving such a problem, for example, as disclosed in Japanese patent application Laid-Open No. 2000-175029, there is a method whereby an image area portion is thinned out at a predetermined ratio and the toner amount is reduced.